Urea Synthesis
Urea is generally produced from ammonia and carbon dioxide. It can be prepared by introducing an ammonia excess together with carbon dioxide at a pressure between 12 and 40 MPa and at a temperature between 150° C. and 250° C. into a urea synthesis zone. The resulting urea formation can be presented best in the form of two consecutive reaction steps, in the first step ammonium carbamate being formed according to the exothermic reaction:2NH3+CO2→H2N—CO—ONH4 after which the ammonium carbamate formed is dehydrated in the second step to give urea according to the endothermic equilibrium reaction:H2N—CO—ONH4H2N—CO—NH2+H2O
The extent to which these reactions take place depends among other things on the temperature and the ammonia excess used. The reaction product obtained in a urea synthesis solution substantially consists of urea, water, unbound ammonia and ammonium carbamate. The ammonium carbamate and the ammonia are removed from the solution and are generally returned to the urea synthesis zone. In addition to the above-mentioned solution in the urea synthesis zone, a gas mixture is formed which consists of unconverted ammonia and carbon dioxide together with inert gases, the so called reactor off-gas. The urea synthesis section may comprise separate zones for the formation of ammonium carbamate and urea. These zones may also be combined in a single apparatus.
In a urea stripping plant the decomposition of the ammonium carbamate that has not been converted into urea and the expulsion of the usual ammonia excess largely takes place at a pressure that is essentially almost equal to the pressure in the synthesis reactor. This decomposition and expulsion take place in one or more stripper(s) installed downstream of the reactor, possibly with the aid of a stripping gas such as, for example, carbon dioxide and/or ammonia, and with the addition of heat. It is also possible to apply thermal stripping. Thermal stripping means that use is made exclusively of the supply of heat to decompose ammonium carbamate and remove the ammonia and carbon dioxide present from the urea solution. The gas stream leaving a stripper contains ammonia and carbon dioxide which are condensed in a high-pressure condenser and then returned to the urea synthesis zone.
In a urea stripping plant the synthesis zone is operated at a temperature of 160-240° C. and preferably at a temperature of 170-220° C. The pressure in the synthesis reactor is 12-21 MPa, preferably 12.5-20 MPa. The ammonia to carbon dioxide molar ratio (N/C ratio) in the urea synthesis zone of a stripping plant lies usually in between 2.2 and 5 and preferably between 2.5 and 4.5 mol/mol. The synthesis zone can be carried out in a single reactor or in a plurality of reactors arranged in parallel or series.
After the stripping treatment, the pressure of the stripped urea solution is reduced in a urea recovery section. In a recovery section the non-converted ammonia and carbon dioxide in the urea solution is separated from the urea and water solution. A recovery section comprises usually a heater, a liquid/gas separation section and a condenser. The urea solution entering a recovery section is heated to vaporize the volatile components ammonia and carbon dioxide from that solution. The heating agent used in the heater is usually steam. The formed vapor in said heater is separated from the aqueous urea solution in the liquid/gas whereafter said vapor is condensed in the condenser to form a carbamate solution. The released condensation heat is usually dissipated in cooling water. The formed carbamate solution in that recovery section operated at a lower pressure than the pressure in the synthesis section is preferably returned to the urea synthesis section operating at synthesis pressure. The recovery section is generally a single section or can be a plurality of recovery sections arranged in series.
Urea Finishing
Today's urea production involves relatively clean processes, particularly low in the emission of urea dust and ammonia. However, besides the chemical synthesis of urea, the production of urea on a commercial scale requires that the urea be presented in a suitable solid, particulate form. To this end, urea production involves a finishing step in which a urea melt is brought into the desired particulate form, generally involving any one of prilling, granulation, and pelletizing.
Prilling used to be the most common method, in which the urea melt is distributed in a prilling tower and the droplets solidify as they fall down. However, the end-product is often desired to have a larger diameter and higher crushing strength than the one resulting from the prilling technique. These drawbacks led to the development of the fluidized bed granulation technique, where the urea melt is sprayed on granules that grow in size as the process continues. Prior to the injection in the granulator, formaldehyde is added to prevent caking and to give strength to the end-product.
The air that leaves the finishing section contains urea dust and ammonia. The latter is particularly caused by an unwanted side-reaction in the finishing step, viz. the formation of biuret, i.e. a dimerization of urea, with release of ammonia. Another side-reaction that may occur is hydrolysis of urea, again with release of ammonia. Thus, despite the relatively clean nature of the urea synthesis, the commercial production of urea inevitably goes with the formation of ammonia. This ammonia is normally emitted through the off-gas of the finishing section of a urea plant.
The concentration of urea takes usually place at high temperatures and sub-atmospheric pressures. Usually concentration of the urea solution to the desired moisture content in the anhydrous urea melt takes place in a concentration section comprising one or a sequence of one or more concentrators in series.
Usually the sub-atmospheric pressure needed to concentrate the urea solution to the desired pressures in the concentrators is done by a combination of cooling down the released gases by cooling water and by using steam as a driving force for an ejector to create the sub-atmospheric pressure in the concentrator. Alternatively in certain urea concentrators the sub-atmospheric pressure is created by the application of vacuum pumps.
The concentrator comprises usually a shell and tube heat exchanger and a gas to liquid separator. The urea solution is subjected to the tube side of the heat exchanger and the heating agent, necessary to heat said solution, is subjected to the shell side of that heat exchanger. The heating agent can be process vapor from the above mentioned urea process, hot water or steam. The urea solution phase and the formed vapor phase leaving said heat exchanger is separated in said gas to liquid separator.
The urea melt leaving the concentration is usually conveyed by a pump to the urea finishing section. The urea finishing sections usually used in urea plants for producing the urea end product are urea granulation finishing and urea prilling finishing.
For urea granulation finishing the desired urea concentration in the urea melt to the granulator is in between 95 and 99% by weight. The urea concentration in the urea melt sent to prilling finishing amounts in between 99.6 and 99.9% by weight. The urea melt sent to the finishing section comprises urea, water and small amounts of ammonia. The ammonia concentration in the urea melt sent to said urea finishing section amounts in between 100 and 900 ppm by weight.
The vapor released in the concentrators comprises ammonia, carbon dioxide and water. Said vapor is condensed in a condenser. The heat of condensation is usually dissipated in cooling water.
In another background method, the urea solution leaving the recovery section is subjected to a crystallization section. Crystallization sections are usually used when urea end products are needed that comprise a biuret content smaller than 0.5% by weight. A urea solution is subjected to a crystallizer. In the crystallizer usually operated at sub-atmospheric pressure, urea crystallizes partly. The formed vapor in the crystallizer is condensed in condensers while the urea solution including the formed urea crystals leaving the crystallizer is subjected to a liquid to solid separator. In this liquid to solid separator the bulk of solution is separated from the urea crystals whereafter said crystals are subjected to a centrifuge. In the centrifuge the urea crystals are washed by using a mother liquor comprising a urea water solution in which the biuret content amounts in between 1 and 10% by weight.
The liquid phase is furthermore separated from said crystals whereafter the urea crystals are transported to a re-melter to form a concentrated urea melt at a concentration in between 99.5 and 99.9% by weight that is subjected to the urea finishing section.
In the urea finishing section according prilling a substantially anhydrous urea melt is sprayed from the top of a prilling column in a rising stream of air of ambient temperature in which the droplets solidify to form so-called prills. The crystallization heat to solidify the urea droplets is released by this rising air flow.
The urea melt that is delivered to the fluidized bed of a urea granulation section as used for urea finishing section comprises, apart from urea and water also ammonia. The water present in the urea melt evaporates to a large extent during fluid-bed granulation of urea. Furthermore the urea melt is solidified. The released crystallization heat is in general removed by the air used for keeping the bed fluidized.
By the crystallization of the urea melt, small particles of the formed solidified urea escapes the prilling tower or fluid bed. These small particles are characterized as urea dust with a particle size in between 0.1 and 100 μm dependent of the finishing technology used. These particles leave the finishing by the discharged air flow. Because large amounts of air are used in the described finishing technologies, intense effective dedusting systems are needed to remove said particles from said air flow. Said dedusting systems need high investments and high energy consumptions.
Besides the dust formation in said urea finishing sections a large part of the dissolved ammonia in the urea melt sent to said finishing, is liberated and leaves said finishing section by the discharged air flow too. Dependent of the amount of air used in said finishing section, the ammonia concentration in the discharged air flow varies in between 50 to 150 mg per m3 air.
Environmentally it is not acceptable anymore to sent said formed ammonia and urea dust in the discharged air flow into the atmosphere. Known technologies to remove the ammonia from that air flow, such as acid washing, have the disadvantages that they need high investment cost and the product to be formed by such an acid washing system have to be processed.